The sperm whale is familiar to most people as the "Moby Dick whale." Melville's classic novel Moby Dick famously featured a rogue sperm whale that attacks a whaling vessel. Many people do not know that Moby Dick is based on a true story, the sinking of the whaleship Essex in 1820 by a male sperm whale – and who could blame him? (see In the Heart of the Sea by Nathanial Philbrick for an excellent retelling).

Sperm whales are the largest of the toothed whales. Most of the toothed whales (whales that have teeth in their jaws, like dolphins and orcas) are small to mid-sized, but the sperm whale is a true giant, with large males reaching 65 feet or more in length. This species is sexually dimorphic, as females only grow to about 35 feet in length. Sperm whales are immediately recognizable by their distinctive, enormous, squared-off head, which is up to one-third of the animal's body length. They are well known for their extraordinary diving capabilities; sperm whales can dive almost two miles deep, clear to the ocean floor and for as long as 90 minutes. Why? Because that's where their main prey lives - deep ocean squid.

A sperm whale defecating as it dives. Note muddy looking water to the left under the flukes. This is when we collect it! (Photo: BMMRO)

Sperm whales seem to be quite intelligent, and have a complex social structure. (They have, in fact, the largest brain of any mammal.) Females stay with their mothers for their entire lives. Females will even take turns watching each other's calves at the surface, while the other mothers are off on a deep dive. Males move between these matrilineal groups, apparently looking for mating opportunities, but large males are frequently seen alone. Juvenile sperm whales appear to form small groups and can hang together for several years before reaching sexual maturity.

We have no idea what this whale is doing. It's upside down, at the surface, mouth open. Huh? (Photo: BMMRO)

Sperm whales are very vocal; they're almost constantly producing a stream of different kinds of noises, clicks and clanks and other calls. Some of these calls are used for echolocation (that is, finding prey by means of sound echoes) and some click patterns (called codas) are used for communication. Fortunately for us, all that noisy vocalizing makes it easy for us to determine whether or not there are any sperm whales near our boat. We just drop a hydrophone - an underwater microphone - in the water and listen for a minute. If sperm whales are anywhere nearby, we immediately hear their constant chatter of clicks, clangs and other noises. If the hydrophone is silent, we can be confident there are no sperm whales within miles. (And then we move a few miles further on, to try our luck elsewhere.) This week, our hydrophone has stubbornly remained silent, no matter where we were. We'll see what happens tomorrow!